Hot melt PSA's have been used to bond a variety of materials to many substrates. A pressure-sensitive adhesive is an adhesive that retains high surface tackiness over time. In other words, a PSA has a theoretically infinite open time. The bonding force of the PSA is derived from the ability of the adhesive to be compatible with the surface of both the substrate and the bonded materials. The adhesive bond arises from the compatibility of the adhesive mass with both the object and the substrate and the internal cohesiveness of the adhesive mass. Any change in the nature of the adhesive materials can have a large effect on bond strength and the cohesiveness of the adhesive. Any substantial degradation of the adhesive materials can cause loss of bond strength. Further, adhesive bond strength is reduced by contamination of the adhesive material or interference between surfaces at the interface between the adhesive bond and the adjacent surface.
We have found that the degradation of the adhesive mass and the resulting bond can be caused by electromagnetic radiation such as visible and ultraviolet light. The radiation degrades the polymer species in the adhesive, increases brittleness or substantially lowers molecular weight. Further, the contamination of the adhesive mass by plasticizer migrating from an adjacent surface of a plasticized object can cause substantial adhesive bond reduction.
Plasticizers are used to alter the glass transition temperature of the polymer used in the adhesive which can enhance adhesive compatibility with surfaces. The glass transition temperature (T.sub.g) is a temperature below which free rotation of the polymer ceases due to intramolecular energy barriers. Well-plasticized adhesives retain a solid physical nature while having significant flexibility and surface compatibility. There are generally two types of plasticization. A polymer may be "internally plasticized" or "externally plasticized." An internally-plasticized polymer is plasticized using copolymerized monomers. The plasticizing comonomer lowers the overall T.sub.g and plasticizes the polymer. External plasticizers are added to a polymer to form a plasticized mixture of polymer and plasticizer.
Vinyl resin materials are often heavily plasticized to improve manufacture and use properties. Plasticizers are intimately mixed with the vinyl plastic material. After a period of time under heat or other driving force, plasticizers can physically separate from the vinyl plastic. Both volatile and non-volatile plasticizers can physically migrate from the vinyl material to an adjacent compatible layer. The migration occurs wherein the adhesive system is in contact with a vinyl plastic. The vinyl plasticizer, which is often soluble or compatible with the adhesive, can migrate into the adhesive mass.
Hot melt adhesives are thermoplastic materials that can be applied in melt form, typically free of an aqueous or organic solvent, that when cooled can form a solid bond line. Upon contact with the substrate, the hot melt adhesives can cool, solidify, and form a strong bond between an object and a substrate surface. Hot melt adhesives can be pressure-sensitive depending upon formulation.
Organic molecules that make up the hot melt PSA material can contain groups or bonds which absorb electromagnetic radiation, such as UV light. The energy from the radiation can cause oxidation, further polymerization, depolymerization, crosslinking, and a variety of other reactions. Such reaction in the base polymer or adhesive additive can substantially change the properties of the adhesive mass. The adhesive can lose flexibility, can lose cohesiveness, can have reduced adhesive bond strength, etc. While this UV degradation problem has attracted a great deal of attention, no fully-satisfactory material has been found for use in application subject to exposure to sunlight or other sources of copious amounts of UV light.
Further problems result from plasticizer contamination. Plasticizer present in a typically resinous substrate can migrate from the substrate into the adhesive layer. This migration can be promoted by heat, chemical concentration gradients, relative compatibility or solubility, and environmental conditions. Over time, as the plasticizer migrates into the adhesive, slowly increasing in concentration, the adhesive polymer base becomes diluted, resulting in a weakened adhesive. Thus, there is a need for an adhesive which resists the migration of the plasticizer.
Hattori et al., U.S. Pat. No. 4,734,447, discloses an improved hot melt adhesive composition comprising a styrene block copolymer resin, a tackifier, and a room temperature solid polybutene having a molecular weight greater than 1,000,000. The solid polybutene recited in the Hattori patent is reacted within an organic peroxide, for example, benzyl peroxide, to cleave the polybutene molecule. The Hattori material is not a true pressure-sensitive adhesive since it loses open time after application.
We are aware that PSA's have been formulated with an ABA block copolymer, a tackifying resin, and an oil plasticizer. These adhesive systems can be subject to both UV and plasticizer migration problems. Similar compositions using an unmodified polybutene plasticizer can have plasticizer stability without UV resistance. Similarly, PSA's with an ABA block copolymer, with no plasticizer, blended with liquid resins can be made UV stable but generally lack plasticizer stability.
Lastly, for many years, solvent-based adhesives comprising a major proportion of an organic solvent, typically vinyl polymers and additional adhesive components, have been applied to a variety of substrates for the purpose of displaying PSA properties in adhering the substrate to objects and other substrates. Such solvent-based adhesives suffer the severe disadvantage that many solvents are flammable or objectionable due to toxic or other undesirable effects on workers. Suitable replacements for solvent-based adhesives have been an object of search for many years.
Accordingly, a significant need exists for a hot melt PSA composition which combines, in a single composition, excellent UV stability, resistance to plasticizer migration, and can be used as a substitute for solution-acrylic (solvent-based) and emulsion-acrylic (aqueous-based) adhesives which have long dominated many adhesive applications and uses, including adherent tape, adhesive film, PSA labels, and others.